Method for independent control of polycrystalline silicon-germanium in an HBT

ABSTRACT

In one embodiment a precursor gas for growing a polycrystalline silicon-germanium region and a single crystal silicon-germanium region is supplied. The precursor gas can be, for example, GeH 4 . The polycrystalline silicon-germanium region can be, for example, a base contact in a heterojunction bipolar transistor while the single crystal silicon-germanium region can be, for example, a base in the heterojunction bipolar transistor. The polycrystalline silicon-germanium region can be grown in a mass controlled mode at a certain temperature and a certain pressure of the precursor gas while the single crystal silicon-germanium region can be grown, concurrently, in a kinetically controlled mode at the same temperature and the same pressure of the precursor gas. The disclosed embodiments result in controlling the growth of the polycrystalline silicon-germanium independent of the growth of the single crystal silicon-germanium.

This is a continuation of pending U.S. application Ser. No. 09/667,274filed Sep. 22, 2000 now U.S. Pat. No. 6,365,479.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of fabrication ofsemiconductor devices. More specifically, the invention relates to thefabrication of silicon-germanium semiconductor devices.

2. Background Art

In a heterojunction bipolar transistor, or HBT, a thin silicon-germaniumlayer is grown as the base of a bipolar transistor on a silicon wafer.The silicon-germanium HBT has significant advantages in speed, frequencyresponse, and gain when compared to a conventional silicon bipolartransistor. Speed and frequency response can be compared by the cutofffrequency which, simply stated, is the frequency where the gain of atransistor is drastically reduced. Cutoff frequencies in excess of 100GHz have been achieved for the HBT, which are comparable to the moreexpensive GaAs. Previously, silicon-only devices have not beencompetitive for use where very high speed and frequency response arerequired.

The higher speeds and frequency response of the HBT have been achievedas a result of taking advantage of the narrow band gap forsilicon-germanium. The energy band gap of silicon-germanium is smallerthan it is for silicon, lying between the intrinsic band gap of silicon(1.12 eV) and germanium (0.66 eV). The band gap is reduced further bythe compressive strain in the alloy layer, with the band gap beingreduced even further with increasing germanium content. The narrowerband gap helps to increase the gain of the HBT by facilitating carrierinjection across the emitter-base junction.

It is also known in the art that grading the concentration of germaniumin the silicon-germanium base builds into the HBT device an electricfield, which accelerates the carriers across the base, therebyincreasing the speed of the HBT device compared to a silicon-onlydevice. A reduced pressure chemical vapor deposition technique, orRPCVD, used to fabricate the HBT device allows for a controlled gradingof germanium concentration across the base layer. As already noted,speeds in the range of approximately 100 GHz have been demonstrated forsilicon-germanium devices, such as the HBT.

Because the benefits of a high gain and high speed silicon-germanium HBTdevice can be either partially or completely negated by a high basecontact resistance, it is important that the resistance of the basecontact be kept to an absolute minimum. By way of background, a basecontact may be provided by forming an electrical conductor in contactwith the epitaxial silicon-germanium base region. Such a conductor isusually composed either of a metal or polycrystalline semiconductormaterial. The choice of material is driven by several constraints andconsiderations. For example, it is not feasible to use a metal early inthe fabrication for fear of contamination as well as practicality ofintegration. Also, the geometry of the base region may necessitate acontact of semiconductor material rather than metal, As such, it isoften required to form a contact made of polycrystallinesilicon-germanium to make an electrical connection with the singlecrystal silicon-germanium base. The process of fabricating the epitaxialor vertical transistor profile while simultaneously fabricating thepoly-crystalline external base contact is known as a non-selectiveprocess. In other words, two critical components of the HBT structureare fabricated concurrently. Just as the physical properties of thesingle crystal silicon-germanium base are important in building ahigh-performance silicon-germanium HBT, attaining the optimum physicalproperties of the polycrystalline silicon-germanium material to serve asthe external base contact is equally important to realize the inherentperformance benefit offered by the HBT device.

In order to achieve satisfactorily low resistance in the base contact,it is required to control the thickness and morphology of the basecontact polycrystalline material. At the same time, the processes thatare used to control the attributes of the polycrystalline base contactmaterial must not deleteriously affect the properties of the epitaxialbase itself. Accordingly, a fabrication technique is needed to achieveindependent control of the physical properties of the polycrystallinebase contact, including thickness and morphology, while maintaining thephysical properties of the epitaxial silicon-germanium base, in order toallow the formation of an optimum low-resistive conduction path to thebase of the HBT.

According to one known technique of fabrication of heterojunctionbipolar transistor devices, a relatively high processing temperature isused for silicon-germanium epitaxy in an RPCVD technique. The hightemperature—higher than approximately 700° C.—sacrifices importantcontrol over the thickness and morphology of the base contact and,consequently, the resulting base contact resistance.

Thus, there is need in the art to retain important control over theresulting base contact resistance without sacrificing manufacturingthroughput. There is further need in the art to decrease the basecontact resistance of a heterojunction bipolar transistor deviceproduced in the fabrication process while maintaining the desired dopantand germanium concentration profiles in the HBT. There is also need inthe art to maintain the high throughput in the fabrication process whenreducing the base contact resistance and while maintaining the desireddopant and germanium concentration profiles in the silicon-germaniumbase.

SUMMARY OF THE INVENTION

According to the present invention, important control over theproperties of the base contact in a heterojunction bipolar transistor isachieved while maintaining the desired dopant and germaniumconcentration profiles in the heterojunction bipolar transistor.

In one embodiment of the invention a precursor gas for growing apolycrystalline silicon-germanium region and a single crystalsilicon-germanium region is supplied. The precursor gas can be, forexample, GeH₄. The polycrystalline silicon-germanium region can be, forexample, a base contact in a heterojunction bipolar transistor while thesingle crystal silicon-germanium region can be, for example, a base inthe heterojunction bipolar transistor.

The polycrystalline silicon-germanium region can be grown in a masscontrolled mode at a certain temperature and a certain pressure of theprecursor gas while the single crystal silicon-germanium region can begrown, concurrently, in a kinetically controlled mode at the sametemperature and the same pressure of the precursor gas.

The invention results in controlling the growth of the polycrystallinesilicon-germanium independent of the growth of the single crystalsilicon-germanium. Accordingly, important control over the properties ofthe base contact in the heterojunction bipolar transistor is achievedwhile maintaining the desired dopant and germanium concentrationprofiles in the heterojunction bipolar transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross sectional view of some of the features of anNPN HBT fabricated in accordance with one embodiment of the presentinvention.

FIG. 2 illustrates the relative concentrations of dopants and germaniumas a function of depth after completion of fabrication of an NPN HBT inaccordance with one embodiment of the present invention.

FIG. 3 illustrates silicon growth rates as a function of temperature andprecursor gas pressure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is method for independent control ofpolycrystalline silicon-germanium in a silicon-germanium HBT and relatedstructure. The following description contains specific informationpertaining to the implementation of the present invention. One skilledin the art will recognize that the present invention may be implementedin a manner different from that specifically discussed in the presentapplication. Moreover, some of the specific details of the invention arenot discussed in order to not obscure the invention. The specificdetails not described in the present application are within theknowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely example embodiments of the invention.To maintain brevity, other embodiments of the invention which use theprinciples of the present invention are not specifically described inthe present application and are not specifically illustrated by thepresent drawings.

FIG. 1 shows a cross sectional view of various features and componentsof structure 100 which includes various features and components of anembodiment of the invention as described below. Certain details andfeatures have been left out which are apparent to a person of ordinaryskill in the art. Structure 100 includes collector 104, base 120, andemitter 130. Collector 104 is N-type single crystal silicon which can bedeposited epitaxially using an RPCVD process in a manner known in theart. Base 120 is P-type silicon-germanium single crystal depositedepitaxially in a “nonselective” RPCVD process according to oneembodiment of the invention as described below. As seen in FIG. 1, base120 is situated on top of, and forms a junction with, collector 104.Base contact 121 is polycrystalline silicon-germanium depositedepitaxially in a “nonselective” RPCVD process according to oneembodiment of the invention as described below. Base 120 and basecontact 121 connect with each other at interface 122 between the contactpolycrystalline material and the base single crystal material. Emitter130 which is situated above and forms a junction with base 120 iscomprised of N-type polycrystalline silicon. Collector 104, base 120,and emitter 130 thus form a heterojunction bipolar transistor (“HBT”)which is generally referred to by numeral 150 in FIG. 1.

Buried layer 102, which is composed of N+ type material—meaning that itis relatively heavily doped N-type material—along with collector sinker106 (also referred to as collector plug 106) provide a low resistanceelectrical pathway from collector 104 to a collector contact (thecollector contact is not shown in any of the Figures). Deep trenchstructures 108 and silicon oxide (SiO₂) structures 110 provideelectrical isolation from other devices on silicon substrate 101 in amanner known in the art.

Continuing with structure 100 in FIG. 1, buried layer 102 of N+ typematerial in silicon substrate 101, deep trench structures 108, andsilicon oxide structures 110 are formed in a manner known in the art.Collector sinker 106 is formed by diffusion of heavily concentrateddopants from the surface of collector sinker 106 down to buried layer102. Collector sinker 106 provides part of a low resistance electricalpath from collector 104 through buried layer 102 to the collectorcontact (not shown).

By way of background, characteristics and functionality of HBT 150 areaffected and can be tailored by varying the steps of the fabricationprocess. One useful tool for controlling the resultant performancecharacteristics of HBT 150 is the dopant and silicon-germanium profile.FIG. 2 shows one such profile for HBT 150 fabricated according to oneembodiment of the present invention. It is desirable to accuratelycontrol the dopant and silicon-germanium profile of HBT 150 to achieve adesired HBT performance.

FIG. 2 shows graph 200 having dopant concentration axis 205, in atomsper cubic centimeter. Graph 200 also shows separate germaniumconcentration axis 215, in atoms per cubic centimeter. Both dopantconcentration axis 205 and germanium concentration axis 215 are plottedagainst depth axis 210, measured in nanometers of depth from the topsurface of HBT 150. As seen in graph 200 of FIG. 2, down to depth ofapproximately 190.0 nanometers corresponds to emitter 130 of HBT 150.This depth, i.e. the depth corresponding to emitter 130, is marked bydashed line 230 in graph 200 to indicate that the area between dashedline 230 and dopant concentration axis 205 corresponds to emitter 130 inFIG. 1.

Also, as seen in graph 200 of FIG. 2, from depth of approximately 190.0nanometers down to depth of approximately 325.0 nanometers correspondsto the base of HBT 150, This base was shown as base 120 in HBT 150 inFIG. 1. This depth, i.e. the depth corresponding to base 120, is markedby dashed line 230 and by dashed line 233 to indicate that the areabetween dashed line 230 and dashed line 233 corresponds to base 120 inFIG. 1.

Further, as seen in graph 200 of FIG. 2, from depth of approximately325.0 nanometers downward corresponds to the collector of HBT 150. Thecollector was shown as collector 104 in HBT 150 in FIG. 1. This depth,i.e. the depth corresponding to collector 104, is marked by dashed line233 to indicate that the area between dashed line 233 and germaniumconcentration axis 215 corresponds to collector 104 in FIG. 1.

Germanium concentration curve 220 represents the concentration profilefor germanium, and corresponds to germanium concentration axis 215.Boron concentration curve 222 represents the concentration profile forboron, and corresponds to dopant concentration axis 205. Arsenicconcentration curve 224 represents the concentration profile forarsenic, and also corresponds to dopant concentration axis 205.

By understanding characteristic growth rates for silicon andsilicon-germanium according to temperature, pressure, flow rate, and theeffect of doping and dopants, as well as the effect of strain resultingfrom the epitaxial growth of silicon-germanium on top of silicon due tothe difference between the two materials, the process of achieving thedesired pre-determined profile can be controlled in order to produce amultilayer collector-base-emitter stack with the desired profile.

Referring to FIG. 1, the portion of multilayer stack structurecomprising collector 104, base 120, and emitter 130 is formed as aresult of several processes. Collector 104 can be formed by epitaxialdeposition of silicon over silicon buried layer 102. Formation ofcollector 104 includes arsenic doping which results in an N-type layer.As stated above, the collector region is shown in graph 200 as theregion confined between dashed line 233 and germanium concentration axis215. By referring to dopant concentration axis 205 and arsenicconcentration curve 224, it is seen that arsenic atoms have aconcentration of approximately 5*10¹⁸ atoms per cubic centimeter in thecollector region. It is also seen from graph 200 that boron atoms have anegligible concentration in the collector region. Accordingly, collector104 is an “N-type” single crystal silicon.

A silicon seed layer is placed on top of collector 104 to maintain goodcrystallinity to aid growth of silicon-germanium above collector 104.Silicon-germanium, which will form part of base 120, is grown by epitaxyon top of the silicon seed layer. The concentration of germanium in thesilicon-germanium layer comprising base 120 is graded by depth in thelayer.

As stated above, base region is shown in graph 200 as the regionconfined between dashed line 230 and dashed line 233. Thus, in thepresent embodiment, the base region covers a depth of approximately190.0 nanometers down to approximately 325.0 nanometers. By referring todopant concentration axis 205 and arsenic concentration curve 224 ingraph 200 it is seen that arsenic atoms have a concentration ofapproximately 5*10¹⁸ atoms per cubic centimeter in the base region. Byreferring to dopant concentration axis 205 and boron concentration curve222, it is also seen that boron atoms have a concentration ranging fromapproximately 1*10¹⁶ to 1*10¹⁸ atoms per cubic centimeter. Thisconcentration of boron atoms renders base 120, which is confined to adepth of approximately 190.0 nanometers to a depth of approximately325.0 nanometers, a “P-type” base.

Dashed line 230 in graph 200, which marks the upper end of base 120 at adepth of approximately 190.0 nanometers in HBT 150, also corresponds tothe emitter-base junction of HBT 150. A single crystal “silicon cap”occupies the region from slightly below the emitter-base junction, whichis at a depth of approximately 190.0 nanometers, to a depth ofapproximately 150.0 nanometers indicated by dashed line 231 in graph200, which is inside the emitter region.

From a depth of approximately 150.0 nanometers down to a depth ofapproximately 190.0 nanometers, i.e. within the silicon cap, thegermanium concentration is 0.0%. The germanium concentration increasesfrom 0.0% at a depth of approximately 190.0 nanometers to approximately8.0% at a depth of approximately 250.0 nanometers as seen from germaniumconcentration curve 220. As further seen from germanium concentrationcurve 220, the germanium concentration remains relatively constant froma depth of approximately 250.0 nanometers to a depth of approximately300.0 nanometers. From a depth of approximately 300.0 nanometers, thegermanium concentration decreases from approximately 8.0% toapproximately 0.0% at a depth of approximately 325.0 nanometers as shownby germanium concentration curve 220.

Thus, base 120 of HBT 150 comprises a graded single crystalsilicon-germanium layer occupying a depth of from approximately 190.0nanometers down to approximately 325.0 nanometers. As stated above, thegraded single crystal silicon-germanium layer has a concentrationranging from approximately 0.0% to approximately 8.0%. Thesilicon-germanium graded layer also includes boron doping as shown byboron concentration curve 222. Because of the relative concentrations ofarsenic and boron, discussed above, the graded single crystalsilicon-germanium layer of base 120 is a P-type material.

The Emitter region is shown in graph 200 as the region confined betweendashed line 230 and dopant concentration axis 205. Thus, in the presentembodiment, the emitter region covers a depth of approximately 100.0nanometers down to approximately 190.0 nanometers. It is seen fromarsenic concentration curve 224 in graph 200 that arsenic atoms have aconcentration ranging from approximately 5*10¹⁸ to approximately 5*10²⁰atoms per cubic centimeter in emitter 130 of HBT 150. It is also seenfrom boron concentration curve 220 in graph 200 that boron atoms have aconcentration ranging from approximately 1*10¹⁷ to 5*10¹⁷ atoms percubic centimeter. This concentration of boron atoms in the emitter ismuch smaller than the concentration of arsenic atoms in the emitter. Assuch, the emitter region is an “N-type” region. The base-emitterjunction, identified by dashed line 230 in graph 200, occurs just abovethe lower end of the single crystal silicon cap. As stated above, thesingle crystal silicon cap, which includes the base-emitter junction,spans from a depth slightly below dashed line 230 and up to dashed line231.

By way of background, during a chemical vapor deposition (“CVD”) processused for epitaxially growing silicon-germanium, a gas containing aprecursor for germanium and similarly for silicon flows across thesilicon surface. For CVD processes, hydrides are used as theseprecursors. For example, for germanium the precursor is GeH₄. Theprecursor, such as GeH₄, is subjected to high temperatures. Theprecursor molecule, in this example GeH₄, usually attaches to anavailable silicon site. The germanium-hydrogen bond of the precursorhydride, at high enough temperature, will break apart. So given enoughheat energy the hydrogen-germanium bonds break, the hydrogen isdesorbed, and the germanium atom incorporates into the growing crystal.In one embodiment of the invention precursors containing germanium, forexample GeH₄, as well as precursors containing silicon, for exampleSiH₄, are used to grow an epitaxial silicon-germanium crystal in base120 of the HBT 150.

An important consideration in CVD growth of silicon-germanium is thestrain produced from growing silicon-germanium crystal on top of asilicon crystal. It is desirable to grow silicon-germanium so as to keepthe resulting strain between the silicon and germanium crystals below acritical level. Exposure of the epitaxially grown silicon-germanium tohigh temperature processing may result in partial to full strain reliefvia plastic flow between the silicon and silicon-germanium crystals.Should the strain energy exceed a critical threshold and the thermalprocessing promote the release of stored strain energy, the coherence ofthe epitaxial silicon-germanium structure with the silicon substrate isdegraded. This event is electrically deleterious and would result in aloss of the advantages provided by the silicon-germanium HBT discussedabove.

The requirement that the strain between the silicon substrate andsilicon-germanium crystals should not exceed a critical level alsoresults in a limitation that must be imposed on the thickness andconcentration of germanium of the epitaxial silicon-germanium layer. Forexample, when the thickness of silicon-germanium layer exceeds a certainvalue for a particular germanium concentration, the strain between thesilicon and silicon-germanium crystals would exceed a critical level,which is referred to as a critical thickness. As such, the amount ofacceptable strain would define the critical thickness of thesilicon-germanium layer. It is noted that an increase in germaniumconcentration also increases the strain between the silicon andsilicon-germanium crystals. As such, it is important to maintainaccurate control over processes used to grow the silicon germanium layerin base 120 independent of control over process parameters necessary tofabricate base contact 121 for HBT 150.

During the manufacturing process, the standard high temperatureprocessing steps should not cause the strain between the silicon andsilicon-germanium crystals to exceed a critical level. The higher theconcentration of germanium, the lower is the operating temperature thatneeds to chosen to ensure the strain between the silicon andsilicon-germanium crystals does not exceed a critical level. During thestandard high temperature processing, excess dopant diffusion may alsooccur which would impair the silicon and silicon-germanium junction.

The present invention maintains the fabrication processing temperatureslow to ID ensure the strain between the silicon and silicon-germaniumcrystals does not exceed a critical level. As described below, oneembodiment of the present invention allows an 8.0% concentration ofgermanium without increasing the strain between the silicon andsilicon-germanium crystals beyond a critical level. This is achievedwhile control over physical and electrical properties of the basecontact is maintained because the fabrication processing temperaturesare maintained low. This in turn results in maintaining higher yields inthe manufacturing environment.

In one embodiment of the invention, the epitaxial deposition ofsilicon-germanium described above for forming base 120 is performed in a“nonselective process.” In a nonselective process only a very smallportion of the total area of the chip, for example 1.0%, has singlecrystal area exposed and suitable for epitaxial growth. The remainder ofthe area is covered with other layers, including but not limited tocommon masking materials such as amorphous silicon, for example SiO₂ orSi₃N₄. The nature of the precursors being used is such that depositionwill occur not only on the exposed single crystal area, but also on themasking materials and other layers, which are amorphous. By way ofcontrast, selective area epitaxy is produced in situations whereprecursors can be chosen such that deposition will continue on anexposed semiconductor but not on amorphous silicon or common maskingmaterials. Thus, in the nonselective process, as material is grownepitaxially in the single crystal region of base 120, other material isbeing deposited elsewhere on the amorphous surfaces. The materialdeposited on the amorphous surfaces, however, is not single crystal instructure, but either amorphous or polycrystalline. As discussed in moredetail below, during the nonselective process of epitaxial growth usedin one embodiment of the invention, base contact 121 is formed as apolycrystalline silicon-germanium material.

It is important, for reasons detailed above, that polycrystalline basecontact 121 provide a low resistance connection to base 120. Theimportance of the formation of a low resistance base contact imposesseveral constraints on the resulting morphology of polycrystalline basecontact 121. Moreover, desirable characteristics of a polycrystallinematerial include that it be conformal and smooth so that it is easy todeposit high integrity layers on top of the polycrystalline material.Furthermore, the thickness of the polycrystalline layer must becontrolled. These constraints can be met by controlling variousconditions in the polycrystalline deposition process, such astemperature, pressure, and precursor gas flow rate. However, prior tothe present invention, it was difficult to control the polycrystallinedeposition independently and the resulting properties of the depositedpolycrystalline without affecting, often adversely, the profile andproperties of the epitaxially grown single crystal silicon-germaniumbase.

Previously known methods using nonselective deposition were controlledby the constraints on the epitaxial single crystal silicon-germaniumgrowth in base 120 of HBT 150, with the condition that whatever resultswere achieved for the polycrystalline characteristics had to beaccepted. Otherwise, if it was desired to change the polycrystallinecharacteristics, the single crystal epitaxial silicon-germaniumproperties and profile were subject to change.

In general, maintaining the desired silicon-germanium profile andproviding a low resistance base contact are two concerns in fabricationof HBT 150. In the nonselective epitaxial deposition ofsilicon-germanium used in one embodiment of the invention, singlecrystal growth in the exposed areas of silicon can occur in both a “masscontrolled” growth mode and a “kinetically controlled” growth mode.Also, polycrystalline growth in the non-exposed areas can occur in boththe “mass controlled” growth mode and the “kinetically controlled”growth mode.

Graph 300 in FIG. 3 provides a framework for understanding the differentmass controlled and kinetically controlled growth modes. The kineticallycontrolled growth mode is also known in the art as “site limited growthmode,” and the mass controlled growth mode is also known in the art as“reactant limited growth mode.” Graph 300 includes silicon growth rateaxis 305, in angstroms per minute. Silicon growth rate axis 305 isplotted against temperature axis 310 measured in degrees Celsius. It isnoted that the temperature indicated by axis 310 decreases in thedirection away from the origin of graph 300.

Graph 300 describes silicon growth rate as a function of temperature andprecursor gas pressure for the particular example of an RTCVD processfor epitaxial deposition of single crystal silicon. The same type ofmeasurements may be made for other CVD processes and for polycrystallineas well as single crystal growth. For example, measurements may be madefor the RPCVD used in the embodiment of the present invention describedhere. For epitaxial single crystal growth in the RPCVD example, themeasurements would produce a graph similar to graph 300 but withdifferent specific numerical values for the pressure and for the growthrate on silicon growth rate axis 305 and for temperature axis 310. Forpolycrystalline growth in the RPCVD example, the measurements wouldproduce yet another graph similar to graph 300 but with still differentspecific numerical values for the pressure and for the growth rate onsilicon growth rate axis 305 and for temperature axis 310. The specificnumerical values for the growth rate, temperature, and pressure thusdepends on the type of CVD and whether single crystal or polycrystallinematerial is being grown. However, the general behavior of growth rate asa function of temperature or pressure has a pattern similar to thatshown in graph 300, independent of the type of CVD and whether singlecrystal or polycrystalline material is being grown. It is also notedthat growth of germanium using an appropriate precursor gas, such asGeH₄, behaves similar to the silicon growth shown in graph 300.Therefore, graph 300 is intended for use as a general guide toillustrate the concepts and, as such, no specific numerical values havebeen indicated in graph 300.

At lower temperatures, growth rate is a strong function of thetemperature value, so that the amount of the precursor gas and theprecursor gas pressure are less determinative of growth rate. This iscalled the kinetically controlled growth mode, since the growth isdependent primarily on temperature. Kinetically controlled growth modeis shown in graph 300 as line 330.

As the temperature increases, the growth rate becomes a strongerfunction of the amount of the precursor gas and the precursor gaspressure. In other words, the growth rate becomes a stronger function ofthe amount of the precursor gas present and the precursor gas pressure.Conversely, the growth rate becomes a weaker function of temperature.This is the mass controlled growth mode. Mass controlled growth mode isshown in graph 300, for example, as line 340, for growth at a lowerpressure, “1st pressure.” Mass controlled growth mode is also shown ingraph 300, for example, as line 350, for growth at a higher pressure,“2nd pressure.” The two examples depicted by line 340 and line 350,illustrate that growth rate in the mass controlled growth mode increaseswith increasing pressure.

A simplified illustration of each of the mass controlled and kineticallycontrolled growth modes can be provided by considering the primarylimitation in each growth mode on the chemical reaction comprisingdeposition. In the mass controlled growth mode, there is sufficient heatenergy to deposit substantially all the germanium (or silicon) beingcarried by the precursor gas. Deposition in the mass controlled growthmode is thus limited by the mass and pressure of the precursor gas.Conversely, in the kinetically controlled growth mode, there is notsufficient heat energy to deposit all the precursor gas provided.Deposition in the kinetically controlled growth mode is thus limited bythe breaking up or desorption of hydrogen from the silicon-germaniumcrystal, i.e. by the kinetic energy (i.e. the heat energy) available tobreak the chemical bonds in the precursor gas.

For the embodiment of the present invention described here, singlecrystal silicon-germanium base 120 is grown by epitaxial depositionprocess in the kinetically controlled growth mode. As discussed above,crystal growth in this mode is relatively insensitive to the pressureand precursor gas flow rate. This mode may be operated at a relativelywide range of temperatures, but an upper limit for the temperature isdetermined by the tendency of higher temperatures to cause strain toexceed a critical level, as discussed above.

For the embodiment of the present invention herein described,polycrystalline silicon-germanium base contact 121 is grown by an RPCVDprocess in the mass controlled growth mode. As stated above,polycrystalline growth of silicon-germanium follows the same fundamentalbehavior shown in FIG. 3, but the operating values for the temperatureand pressure parameters are shifted. For example, the mass controlledgrowth mode for polycrystalline silicon-germanium occurs at a lowertemperature in comparison with the temperature at which mass controlledgrowth mode for single crystal silicon-germanium occurs.

Thus the transition from the kinetically controlled growth mode to themass controlled growth mode occurs at a lower temperature for thepolycrystalline silicon-germanium. It follows that polycrystallinesilicon-germanium growth may be conducted in the mass controlled growthmode at temperatures which would correspond to the kineticallycontrolled growth mode for single crystal silicon-germanium. The presentinvention takes advantage of the fact that the polycrystalline growth inthe mass controlled growth mode is a stronger function of pressure andprecursor gas flow rate to achieve control over polycrystallinesilicon-germanium growth in base contact 121 independent of the singlecrystal silicon-germanium growth in base 120. In other words, thepolycrystalline silicon-germanium growth can occur without causingsubstantial growth in base 120 at lower temperatures. Thus, at lowertemperatures, for example at 650° C., the invention achieves growth ofpolycrystalline silicon-germanium in base contact 121 without causing asubstantial growth in single crystal silicon-germanium base 120.

At higher pressure and temperature, more polycrystalline growth occurs.At lower pressure and temperature, there is more amorphous deposition.For pressures in the range of approximately 200.0 Torr down toapproximately 100.0 Torr, the growth is polycrystalline. Atapproximately 75.0 Torr, the growth becomes more amorphous. Thedesirability of achieving more polycrystalline than amorphous growthsets a lower limit on the operating temperature. The present embodimentof the invention uses a temperature of approximately 650° C. and apressure of approximately 100.0 Torr. At approximately 650° C. at 100.0Torr, single crystal silicon-germanium growth in base 120 of HBT 150 issoundly in the kinetically controlled growth mode and occurs at a muchlower rate compared with the growth of polycrystalline silicon-germaniumin base contact 121 which occurs at a much higher rate in the masscontrolled mode.

Under conditions where the single crystal growth is in the kineticallycontrolled growth mode and polycrystalline growth is in the masscontrolled growth mode, it is possible to supply the precursor gas at awide range of volume. In the particular embodiment of the inventiondescribed here, it is possible to vary precursor gas in a range fromapproximately 100.0 SCCm (standard cubic centimeters) up toapproximately 400.0 SCCm. For precursor gas flow rates in this range,the epitaxial growth rates for the single crystal silicon-germanium arenot significantly affected. The polycrystalline growth rates, on theother hand, vary almost linearly as a function of the precursor gas flowvolume in this range.

Thus, the present invention achieves control over the ratio ofpolycrystalline silicon-germanium base contact 121 deposition rate tosingle crystal silicon-germanium base 120 deposition rate. This ratio isreferred to as “the deposition ratio” in the present application. At thetemperatures and pressures previously used, approximately 725° C. at100.0 Torr, the deposition ratio of single crystal base 120 topolycrystalline base contact 121 was approximately one to one. That is,the single crystal material and the polycrystalline material depositedat nearly the same rate. The present invention achieves control over thedeposition ratio and achieves a one to two deposition ratio, i.e. thepolycrystalline silicon-germanium in base contact 121 grows twice asfast as the single crystal silicon-germanium in base 120. Further, theproperties of polycrystalline deposition can be independently controlledby suitable choices of physical conditions of the process includingtemperature and pressure.

The properties of the polycrystalline deposition over which control isgained include the thickness of the deposition and also the structure interms of how much of the deposition is polycrystalline and how much isamorphous or unstructured deposition. These two properties directlyaffect the resistance of the polycrystalline deposition and therebypolycrystalline base contact 121. Increased thickness lowers theresistance and also improves the crystal structure gained by increasingthe proportion of polycrystalline material over amorphous. The controlof properties of the polycrystalline deposition for polycrystalline basecontact 121 results in maintaining the advantages of silicon-germaniumHBT 150. In other words, the advantageous properties of HBT device 150including the gain, speed, and frequency response are maintained.

For application to the particular HBT structure 150 herein described,the temperature and pressure conditions were approximately 650° C. at100 Torr. By application of the invention to HBT 150, the base contactresistance is reduced from values of approximately 1000 ohms permicrometer to approximately 600 ohms per micrometer, i.e. a 40%reduction.

As described above, present invention allows an 8.0% concentration ofgermanium without increasing the strain between the silicon andsilicon-germanium crystals beyond a critical level. Accordingly, theinvention maintains higher yield in the manufacturing environment sincethe strain does not exceed a critical level. This is achieved whilecontrol over physical and electrical properties of the base contact ismaintained.

It is appreciated by the above detailed disclosure that the inventionprovides a method for controlling the deposition of polycrystallinematerial independently of the deposition of single crystal material in asilicon-germanium nonselective epitaxial process. Using the invention,various important properties of the base contact in an HBT can becontrolled and improved. Although the invention is described as appliedto the construction of a heterojunction bipolar transistor, it will bereadily apparent to a person of ordinary skill in the art how to applythe invention in similar situations where control is needed ofpolycrystalline properties of silicon-germanium independent of effectson single crystal silicon-germanium growth.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. For example, although theCVD process used in the particular embodiment of the present inventiondescribed here is the reduced pressure, or RPCVD, other types of CVDtechniques known in the art could be used without departing from thescope of the present invention. These types of CVD techniques mayinclude ultra high vacuum CVD (UHVCVD), atmospheric pressure CVD(APCVD), and rapid thermal CVD (RTCVD). Moreover, while the inventionhas been described with specific reference to certain embodiments, aperson of ordinary skills in the art would recognize that changes can bemade in form and detail without departing from the spirit and the scopeof the invention. The described embodiments are to be considered in allrespects as illustrative and not restrictive. It should also beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of many rearrangements,modifications, and substitutions without departing from the scope of theinvention.

Thus, method for independent control of polycrystallinesilicon-germanium in a silicon-germanium HBT and related structure havebeen described.

What is claimed is:
 1. A method comprising steps of: determining atemperature at which a precursor grows at a kinetically controlledgrowth mode over a first area and at a mass controlled growth mode overa second area; supplying said precursor at said temperature so as togrow a single crystal region over said first area and a polycrystallineregion over said second area.
 2. The method of claim 1 wherein saidfirst area comprises an exposed area of silicon and said second areacomprises a non-exposed area.
 3. The method of claim 1 wherein saidprecursor comprises germanium and hydrogen.
 4. The method of claim 1wherein said single crystal region comprises single crystalsilicon-germanium and said polycrystalline region comprisespolycrystalline silicon-germanium.
 5. The method of claim 1 wherein saidsingle crystal region is in contact with said polycrystalline region. 6.The method of claim 1 wherein said single crystal region is a base in aheterojunction bipolar transistor.
 7. The method of claim 1 wherein saidpolycrystalline region is a base contact in a heterojunction bipolartransistor.
 8. The method of claim 1 wherein said temperature isapproximately 650° C.
 9. The method of claim 1 wherein said precursor issupplied at a gas pressure promoting said kinetically controlled growthmode over said first area and said mass controlled growth mode over saidsecond area.
 10. The method of claim 9 wherein said gas pressure isapproximately 100 Torr.
 11. The method of claim 1 wherein saidpolycrystalline region grows approximately twice as fast as said singlecrystal region.
 12. A method comprising steps of: growing a singlecrystal silicon-germanium base in a kinetically controlled growth modeat a first temperature and a first precursor gas pressure, said singlecrystal silicon-germanium base having a junction with a collector;growing a polycrystalline silicon-germanium base contact in a masscontrolled growth mode concurrently with said single crystalsilicon-germanium base at said first temperature and said firstprecursor gas pressure, said polycrystalline silicon-germanium basecontact being in electrical contact with said single crystalsilicon-germanium base; growing an emitter second junction with saidsingle-crystal silicon germanium base.
 13. The method of claim 12further comprising the step of supplying a precursor gas at said firsttemperature and said first precursor gas pressure.
 14. The method ofclaim 13 wherein said first precursor gas comprises germanium andhydrogen.
 15. The method of claim 12 wherein a deposition ratio of saidpolycrystalline silicon-germanium base contact to said single crystalsilicon-germanium base is approximately 2 to
 1. 16. The method of claim12 wherein said first temperature is approximately 650° C.
 17. Themethod of claim 12 wherein said first precursor gas pressure isapproximately 100 Torr.
 18. The method of claim 12 wherein said singlecrystal silicon-germanium base comprises approximately 8% germanium andapproximately 92% silicon.
 19. The method of claim 12 wherein saidpolycrystalline silicon-germanium base contact has a base contactresistance value of approximately 400 ohms.
 20. The method of claim 12wherein said single crystal silicon-germanium base is grown over anexposed area of silicon and said polycrystalline silicon-germanium basecontact is grown over a non-exposed area.